The influences of different alkaline conditions on the kinds and morphologies of steel slag’s hydration products, Ca(OH)2 content of hydration products, pore structure of hardened paste, non-evaporable water content of hydration products, and strength of steel slag mortar were investigated by changing the initial alkalinity of the hydration condition of steel slag. The results showed that increasing the initial alkalinity of hydration condition can promote the early hydration of steel slag’s active components (e.g., C2S, C3S, and C12A7 ), but it has little influence on their late-age hydration degree. The hydration degree of non-active components (e.g., RO phase and Fe3O4 ) of steel slag is very low even under strong alkaline condition with pH value of 13.8. The excitation effect of alkaline condition on the early hydration of steel slag is more obvious with the increase of pH value, but the kinds of steel slag’s hydration products are not influenced by changing the alkaline condition. The amount of steel slag’s hydration products is limited, so the strength of alkali-activated steel slag mortar is very low though the strong alkaline condition significantly promotes the early hydration of steel slag. Steel slag is not an ideal raw material for alkali-activated cementitious material. The influences of different alkaline conditions on the kinds and morphologies of steel slag’s hydration products, Ca (OH) 2 content of hydration products, pore structure of hardened paste, non-evaporable water content of hydration products, and strength of steel slag mortar were investigated by changing the initial alkalinity of the hydration condition of steel slag. The results showed that increasing the initial alkalinity of hydration condition can promote the early hydration of steel slag’s active components (eg, C2S, C3S, and C12A7) but it has little influence on their late-age hydration degree. The hydration degree of non-active components (eg, RO phase and Fe3O4) of steel slag is very low even under strong alkaline condition with pH value of 13.8. The excitation effect of alkaline condition on the early hydration of steel slag is more obvious with the increase of pH value, but the kinds of steel slag’s hydration products are not influenced by changing the alkaline condition. The amount of steel slag’s hydration products is limited, so the strength of alkali-activated steel slag mortar is very low though the strong alkaline condition significantly promotes the early hydration of steel slag. Steel slag is not an ideal raw material for alkali-activated cementitious material.